Fix #15955 (#16005)

See #15955 (comment)

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -281,7 +281,7 @@ protected override void OnItemsControlChanged(ItemsControl? oldValue)
  var fromControl = from as Control;
 
  if (count == 0 || 
- fromControl is null && direction is not NavigationDirection.First or NavigationDirection.Last)
+ (fromControl is null && direction is not NavigationDirection.First and not NavigationDirection.Last))
  return null;
 
  var horiz = Orientation == Orientation.Horizontal;